Internal combustion engines are provided with cylinders, each one having a piston coupled to rotate a crankshaft. A fuel and air mixture is injected into a combustion chamber of each cylinder and ignited, resulting in hot expanding exhaust gasses causing reciprocal movement of the piston, the fuel being provided by injectors which in turn receive fuel at high pressure from a fuel common rail that is in fluid communication with a high pressure fuel pump.
Internal combustion engines are also generally equipped with an Electronic Control Unit (ECU) and the crankshaft is generally equipped with a crank position sensor suitable to send crankshaft signals to the ECU.
In order to improve the characteristics of exhaust emissions and reduce combustion noise in engines, particularly Diesel engines having a common-rail fuel injection system, a so-called multiple fuel injection pattern is adopted according to which the fuel quantity to be injected in each cylinder at each engine cycle is split into a plurality of injections. Thus, a typical multiple injection pattern may include preliminary injections (also known as pilot injections), which may be in turn split into two or more injection pulses, followed by one or more main injection pulses, followed by a number of after and post injection pulses.
The pilot injection pulses have an effect both on the level of combustion noise and exhaust emissions, and their duration or energizing time (ET) is generally mapped in memories of the electronic injection control unit. The mapped values of the energizing time are predetermined with reference to an injection system having nominal characteristics, i.e. components having no drifts.
However, the fuel quantity which is actually injected by an injector into the corresponding engine cylinder is inevitably affected by drifts, with respect to the desired or nominal value and this fact, during the vehicle lifetime, causes a variation of the combustion noise and exhaust emission characteristics.
Therefore fuel compensation strategies are used to correct the injected fuel quantity in a combustion engine during injector lifetime and periodically adjust the injector energizing time in order to have repetitive performance and accuracy in the fuel injected quantity along the life of the injector.
Also, pilot injections are in the range of a strong non-linearity of the injector performance and therefore more in need of being subjected to a compensation strategy.
To correct the fuel injected quantity, the injector energizing time strategy runs during engine overruns, for example when the automotive vehicle's driver releases the pressure on the accelerator pedal.
During these overruns, the Electronic Control Unit of the engine commands a calibratable test injection (e.g. 1 mm3 of fuel) in one of the cylinder of the engine, while the other injectors are de-energized.
The injected fuel quantity is proportional to the crankshaft acceleration, and injector energizing time strategies process crankshaft timing in order to obtain a signal that is proportional to the acceleration and so to the injected quantity.
The crankshaft acceleration signal is processed taking into account the following considerations.
In an internal combustion reciprocating engine, the gas-pressure torque in each cylinder is a periodic function due to the thermodynamic cycle. In a 4-stroke engine the gas-pressure torque has a period of 720° CR (frequency 0.5 w). Therefore the gas-pressure torque in a 4-stroke engine can be expressed by means of a Fourier's series having fundamental frequency 0.5 w (so the frequencies involved in the series are 0.5 w, 1.0 w, 1.5 w, 2.0 w, 2.5 w, 3.0 w, etc. . . . ). The fundamental frequency having frequency 0.5 w is called component of order 0.5. It has a period of 720° CR.
The injector energizing time signal processing strategy selects the order 0.5 (injection order for an engine where only 1 cylinder is firing and the others are not firing, meaning that there is 1 firing event each 720° CR) and compares this signal to a predefined threshold in order to detect the correct injector energizing time for the pilot injected fuel quantity.
However crankshaft signal is very sensitive to noise, and noise frequency can be dependent from the driveline. From experience, especially in case of higher gears, the characteristic driveline resonance frequency is very close to the firing order (0.5) selected for the injector energizing time learning strategy; this generates errors in the correct energizing time estimation with negative effects on noise, emissions and drivability, and, in some cases, leads to the deactivation of the learning strategy.
From an analysis on different drivelines and applications to detect the noisy resonance frequency of the high gears, it is possible to obtain the following results for the gear dependent noisy frequency: